Process of coating a gas turbine engine alloy substrate

ABSTRACT

A method of coating a gas turbine engine alloy substrate comprising depositing a rare earth and aluminum-containing alloy initial layer to a thickness sufficient to produce and maintain an adherent irregular aluminum oxide, mechanically working the surface of the initial layer to induce irregularity and angular topography in the aluminum oxide to be produced, oxidizing the initial layer to produce a sufficiently thick and irregular aluminum oxide layer to establish mechanical adherence of a noble metal layer and prevent alloying between the initial layer and the noble metal layer, depositing a noble metal layer on the oxidized layer to a thickness of approximately 0.1-0.2 mils and oxidatively treating the coated substrate to cause additional growth of the oxide layer to metallurgically insulate the noble metal layer from the substrate and the initial metal layer.

United States Patent 1 Dils PROCESS OF COATING A GAS TURBINE ENGINEALLOY; SUBSTRATE Ray R. Dils, Madison, Conn.

United Aircraft Corporation, East Hartford, Conn.

Filed: Aug. 6, 1973 Appl. No.: 386,266

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 2/1953 Weinrich 117/71 X l/1961Greene et a1. 117/131 X 9/1961 Hanink et al. 117/131 X 11/1964 Freemanet a1. 117/13ORX 4/1966 .laremus et a1 29/573 X 1111970 Talboom etal..,.... 29/1835 7/1972 Evans et a]. 29/194 10/1972 Kanter 117/131 X 1June 17, 1975 3,754,903 8/1973 Goward et a1. 751m FORElGN PATENTS ORAPPLICATIONS 854,570 ll/1960 United Kingdom 29 573 PrimaryExaminer-Thomas .1. Herbert, Jr. Assistant Examiner-Bruce H. HessAttorney, Agent, or Firmlohn D. Del Ponti [57] ABSTRACT A method ofcoating a gas turbine engine alloy sub strate comprising depositing arare earth and aluminum-containing alloy initial layer to a thicknesssufficient to produce and maintain an adherent irregular aluminum oxide,mechanically working the surface of the initial layer to induceirregularity and angular topography in the aluminum oxide to beproduced, oxidizing the initial layer to produce a sufficiently thickand irregular aluminum oxide layer to establish mechanical adherence ofanoble metal layer and prevent alloying between the initial layer and thenoble metal layer, depositing a noble metal layer on the oxidized layerto a thickness of approximately 0.1-0.2 mils and oxidatively treatingthe coated substrate to cause additional growth of the oxide layer tometallurgically insulate the noble metal layer from the substrate andthe initial metal layer.

14 Claims, 11 Drawing Figures 57- 07 11464022 [40 (/V/t/ZT/J/VS'PATENTEDJUN 17 ms SiiEH @R 6% aw @N/Z PROCESS OF COATING A GAS TURBINEENGINE ALLOY SUBSTRATE BACKGROUND OF THE INVENTION The present inventionrelates to the treatment of metals and alloys and more particularlyrelates to a method for coating gas turbine engine components eitherpartially, as in the form of a thin strip array to provide surfacetemperature or surface strain sensors therefor, or completely to provideimproved resistance of the component to high temperature sulfidation oroxidation.

One of the problems facing the gas turbine industry has been the needfor sensors to provide accurate data such as the steady statetemperature of static or rotating components either in or out of the gaspath. The severe operating environment of gas turbine componentspresents particularly difficult problems in view not only of therequirement that the temperature cycling of the engine be withstood butalso that there be compatibility with the substrate component and noperturbation of the airflow near or heat flow to the component. As willbe appreciated, a sensor on a turbine airfoil capable of obtainingaccurate broadband turbine temperatures including those in excess of2000F without perturbing airflow is an important step forward in theart.

SUMMARY OF THE INVENTION The present invention relates to a method ofcoating a nickelbase, cobalt-base or iron-base gas turbine engine alloy.The invention contemplates a method comprising l) depositing a rareearth and aluminum containing alloy initial layer to a thicknesssufficient to produce and maintain an adherent irregular aluminum oxide,preferably NiCrAlY, CoCrAlY or FeCrAlY to a thickness of0.5-5.0 mils,(2) mechanically working the surface of the initial layer to induceirregularity and angular topography in the aluminum oxide to beproduced, preferably by grit blasting or peening, (3) oxidizing themechanically worked initial layer to produce a sufficiently thick andirregular aluminum oxide layer to promote mechanical adherence of anoble metal layer and to prevent alloying between the initial layer andthe noble metal layer, preferably by an oxidation treatment to form anoxide layer 0.050.1 mil thick such as heating, in air, for 70-170 hoursat 1900F, (4) depositing a noble metal layer on the oxidized initiallayer to a thickness to form a noble metal thermocouple, preferablyapproximately 0.l-0.2 mil and (5) oxidatively treating the coatedsubstrate to cause additional growth of the oxidized initial layer tometallurgically insulate the noble metal layer from the substrate andthe initial layer. In the production of surface temperature sensors onthe substrate, the noble metal coating is in the form of a suitable thinstrip array of thermoelectric junctions having thickened end portionssuitable for use as terminal connections.

In the production of surface strain sensors on the substrate, the noblemetal coating is in the form of an array of first and second thin stripelements, the noble metal of the first thin strip element, preferablyplatinum, having a large temperature coefficient of resistivity withrespect to the noble metal of the second thin strip element and thesecond thin strip element, preferably an alloy consisting essentially of8l2 weight percent W, balance Pt, having a large strain coefficient ofresistivity with respect to the first or in the form of an arrayconsisting of the strain sensitive element, 8l2 weight percent W,balance Pt, and a sputtered Pt/Pt-Rh thermocouple located near thecenter of the strain sensitive element. To reduce the rate of oxidationof the Pt-W alloy element above app.oximately l500F. a protective layerof aluminum oxide or calcium stabilized zirconia may be provided,preferably by RF sputtering thereover.

The basic method disclosed herein is particularly useful for overcoatinggas turbine components to provide increased resistance to sulfidation aswell as to high temperature ox dat on. In order to reduce or preventfurther growth of the aluminum oxide layer on the component, an electricfield is superimposed across the aluminum oxide layer with the noblemetal layer as the anode and the substrate as the cathode.

RIEF DESCRIPTION OF THE DRAWINGS An understanding of the invention willbecome more apparent to those skilled in the art by reference to thefollowing detailed description when viewed in light of the accompanyingdrawings, wherein:

FIG. 1(a) is a plan view showing noble metal test elements on a flatdisk;

FIG. 1(b) is a plan view showing an incomplete fourjunction sensor on aflat disk;

FIG. He) is a plan view showing a completed fourjunction sensor on aflat disk;

FIG. 1(d) is a side elevational view of a threejunction sensor array onan erosion bar;

FIG. 2 is a chart showing sensor accuracy;

FIGS. 3(a) and 3(b) are perspective views of a turbine blade havinglarge scale sensor arrays on their surface;

FIGS. 4(a) and 4(b) are diagrammatic plan views of small scale sensorarrays near cooling holes;

FIG. 5 is a diagrammatic plan view ofa two-element strain sensor; and

FIG. 6 is a perspective view, partly cross-sectionally enlarged, ofaturbine component showing the imposition of an electric field across theoxide coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The nickel-base, cobalt-baseand iron-bas gas turbine engine alloys are those strong, hightemperature materials suitable for use in gas turbine engineapplications. Typical of the alloys which may be coated according to thepresent invention are the so-called nickei-base and cobalt-basesuperalloys, viz., those which generally contain 5-25 weight percent Cr,5-15 weight percent Mo, Ta or W and 2-8 weight percent Al and Ti. Alsouseful as substrates are the high temperature iron-base alloys such asthe austenitic stainless steels or Kanthal A (5.5 Al, 22 Cr, balanceFe).

In the production of surface temperature sensors on gas turbine enginecomponents, the first step is the deposition of an initial layer of analloy onto a gas turbine engine alloy substrate. The initial layer isany rare earth or rare earth particle-containing alloy which can form anadherent irregular aluminum oxide. In general, the initial layercontains no more than approximately 2 percent, by weight, of the rareearth metal and approxi- 1.1ately 5-25 percent, by weight, aluminum andpreferably consists of a coating such as NiCrAIY (20-35 weight percentCr, 15-20 weight percent Al, 0.05-0.3 Y, balance Ni), CoCrAlY (19-24weight percent Cr,

13-17 weight percent Al, 0.6-0.9 Y, balance Co), FeCrAlY (25-29 weightpercent Cr, 12-14 weight percent Al, 0.6-0.9 Y, balance Fe) or an alloysuch as 25 Cr, 15 Ni, 5 Ta, 5 A1, 0.1 Y, balance Co. In some cases it isdesirable to use alloys with the same base metal in the substrate andinitial layer, e.g., FeCrAlY with ironbase alloys, CoCrAlY withcobalt-base alloys, etc. However. in general. various combinations maybe utilized. The initial layer thickness must only be thick enough toproduce and maintain an adherent metal oxide, preferably approximately0.5-5 mils. and may be deposited by conventional techniques as bysputtering or evaporation.

The second step in the construction of the coating is the mechanicalworking of the initial layer to induce the growth of an adherent metaloxide with an extremely irregular and angular topography. Thesubsequently deposited noble metal layer is primarily mechanicallybonded to the initial layer and any method of surface preparation whichwill induce the growth of an irregular oxide will promote its mechanicaladherence thereto. Grit blasting of the surface with various sizes ofgrit is considered a satisfactory technique. as is peening.

In the third step of the construction of the sensors, the initial layeris oxidized to produce a sufficiently thick and irregular oxide topromote the mechanical adherence of the noble metal layer, and toprovide a sufficiently small number of paths through the oxide to thesubstrate to eliminate alloying between the initial layer and the noblemetal layer. The oxide layer must be thin enough to permit rapidreoxidation of the specimen, provide oxide dimensions representative ofthe oxides grown in the turbine environment in order not to perturb theheat flow in the system and minimize the reduction of the turbinecomponent life due to the oxidation treatment. It has been found thatoxides from 0.05-0.l mil thick which are grown in air for generally70-300 hours at I900F fulfill the above requirements. However, it willbe appreciated that any oxidation treatment which produces an oxidedimension approximating the above range is considered suitable.

The next step comprises the deposition of a noble metal layer to form anoble metal thermocouple. By noble metal is meant such elements oralloys as those of platinum, rhodium or palladium. Each layer isdeposited, preferably by sputtering, to a thickness sufficient to bestable and durable in harsh environments yet thin enough to permitoxygen diffusion through the layer in order to insulate it during thesubsequent oxidation treatment described below. It has been establishedthat noble metal thicknesses between 0.1 and 0.2 satisfy theserequirements. The adherence of the noble metal layer increases withincreasing sputtering substrate temperature. However, some uses ofsensors require high resolution spacial distributions of thermocouplefunctions on the surface of a component and these sensor arrays are bestobtained by low temperature masking procedures. Thus the entire range ofsputtering substrate temperatures, from room temperature to the meltingpoint of the substrate may be utilized.

The fifth step in the process is the electrical insulation of the noblemetal layer from the substrate and the initial layer. It has been foundthat oxidation in air for approximately 30 hours at I900F is sufficientto achieve this result.

The last step in the construction of a sensor is the formation ofrelatively thick terminals at the ends of the sputtered noble metalleads to permit lead wires to be directly connected, as by spot welding.thereto. Terminal thicknesses between 0.2-0.5 mil are sufficient toobtain a durable connection between the sensors and 0.003 mil diameterlead wires. The width of the terminals is smaller than the originalwidth of the sputtered leads to prevent loss of the electricalinsulation of the sensor.

As discussed in the following specific example, surface temperaturesensors made according to the present invention have provided metalsurface temperatures from 0F to the melting point of several nickel-,cobaltor iron-base alloys with the accuracy of special gradeplatinum/platinum-rhodium thermocouples. The presence of the sensor onthe surface of a component did not significantly perturb the heat flowfrom the gas stream to the component or the heat flow within thecomponent. The bandwidth of information was limited only by the relativeamplitudes of the signal and the equivalent input noise of theassociated electronics. In general, useful information may be receivedover a several kilohertz bandwidth. Tests indicated that the sensors areusually durable.

Example Simple sensor elements 10 on flat disks l2 and an erosion bar 14are shown in FIG. 1. The flat disks comprised a substrate of thenickel-base alloy B1900 (nominal composition, by weight percent, 8 Cr,10 Co, 1 Ti, 6 Al, 6 Mo, 0.l l C, 4.3 Ta, 0.15 B, 0.07 Zr, balance Ni)and a sputtered initial layer three mils thick of FeCr- AlY which hadbeen mechanically worked by a No. 320 grit blast and subsequentlyoxidized in air for hours at 1900F to grow an aluminum oxide 0.1 milthick. Platinum test elements 16, 40 mils wide, 750 mils long and 0.lmil thick were sputtered on the flat disk 12 shown in FIG. 1(a). Theplatinuml 0 weight percent rhodium elements 18 of a fourjunction sensorarray were sputtered on the flat disk as shown in FIG. 1(b) followed bythe sputtering of a platinum element 20 across the Pt-10 percent Rhelements 18 to complete the sensor as shown in FIG. 1(c). FIG. 1(d)shows a three-junction sensor array on an erosion bar. As will beappreciated, any desirable array of thermoelectric junctions can besputtered on a turbine component. Since the initial layer coating andoxide are common to high temperature turbine components, the only realchange in the component configuration is due to the sputtered noblemetal layer having a thickness of 0.0001-0.0002 inch. The platinum andplatinumrhodium elements of FIG. 1 do not significantly affect the heatflow from the gas stream to the component or the heat flow within thecomponent. For example, the narrowband (steady state) thermal impedanceof a 0.000! inch platinum element is 2.14 X 10 of the boundary layerimpedance when h 1000 BTU/ft hrR. In addition, the thickness of thenoble metal layer is small with respect to the thickness of the boundarylayer and therefore does not alter the structure of the boundary layer.The platinum element narrowband impedance is 7.5 X 10 of the impedanceof a 0.050 inch section of a nickel-base alloy. The broadband responsenear the turbine component surface is limited by the oxide layer. At 15khz, a harmonic temperature wave travelling across a 0.0001 inch oxidelayer is attenuated to He of the initial amplitude of the wave at thesurface. At the same frequency, the reduction in wave amplitude acrossthe sensor is less than six percent. Therefore, the sensor elements donot affect narrowband or broadband measurements; the useful bandwidth ofthe information is determined by the relative amplitudes of the signaland the equivalent input noise of the associated electronics.

It will be appreciated that the width of the sputtered sensors isextremely small, in this case over 300 times smaller than the width ofconventional thermocouples used by placement in slots in airfoils tomeasure temperatures near the airfoil surfaces. The sputtered sensorwidth is over I times smaller than the conventional strain andtemperature sensors presently applied externally to airfoil surfaces.

The measurement errors of sputtered sensors of the present inventionwere maintained within the limits of error for special grade Pt/Pt-Rhthermocouples. A comparison of the thermoelectric voltage generated by asputtered Pt/Pt-IO percent Rh sensor like the one shown in FIG. 1(c) anda special grade Pt/Pt-IO percent Rh thermocouple is presented in FIG. 2.The specimen was cycled from room temperature to 2000F in randomtemperature intervals for two months. At each temperature, the specimenand standard thermocouple were equilibriated for at least four hoursbefore the temperature was measured. The indicated errors are withinthose expected between different special grade Pt/Ptl0 percent Rhthermocouples. There appear to be no extraordinary errors associatedwith the sputtered sensors.

Durability of the sensors of the instant invention was proven. Duringseveral months of testing, no indication of signal deterioration due toextended exposure at high temperatures was observed. In one experiment asensor was gradually cycled from 2000F to room temperature for twomonths. The specimen holder failed but the sensor itself remainedintact.

Platinum test elements such as those shown in FIG. 1(a) were cycledseveral hundred times from 2000F to room temperature in a stationarygas. The thermal cycling had no effect on the test elements whichremained electrically insulated from the substrate and strongly bondedto the substrate oxide. In another experiment a Pt/Pt-IO percent Rhsensor sputtered on a rod was cycled over 5000 times from l800F to roomtemperature in a moderate velocity gas stream (Ma 05). Although thesubstrate was extensively cracked and plastically deformed causing aloss of electrical insulation between the sensor and the substrate, thesensor remained strongly bonded to the substrate oxide.

The sensors of the present invention are able to withstand extensivegradual or rapid plastic deformation. In one series of tests, platinumtest elements sputtered on flat disks such as those of FIG. 1(a) weredeformed approximately IO percent to concave and convex shapes, yetremained attached to the substrate and electrically insulated therefrom.The sensors can be quite heavily scratched or abraded. Even if the unitsare inordinately handled so that a loss of insulation between the sensorelements and the substrate results, they may be re paired by reoxidizingthe components. In one example, a platinum test element was struckrepeatedly with a ballpeen hammer so that the sensor element wasgrounded to the substrate. The element was nevertheless subsequentlyelectrically insulated from the substrate by oxidizing the component for20 hours at 1900F.

Overall, the surface temperature sensors of the present inventionprovide data which cannot be obtained by state-of-the-art techniques ofthe gas turbine industry. The sensor units provide steady statetemperatures of the external surfaces of both static or rotatingcomponents either in or out of the gas path. Sensor arrays to measurelarge-scale span and radial temperature dis tributions are shown inFIGS. 3(a) and 3(b). Sma|lscale sensor arrays to obtain local surfacetemperatures near an individual cooling hole are shown in FIGS. 4(a) and4(b). In either case, the sensors provide the actual surfacetemperatures in the engine and, correspondingly, detailed experimentalevaluations of the present analytical models of heat transfer in theengine.

Due to the rugged nature of the sensors, the units may be applied tosurfaces of details or subassemblies prior to final fabrication steps.For example, internal surface temperatures of a split blade may beobtained by application to the internal surfaces of each half before thehalves are bonded together. Large-scale heat flows in the blade can beobtained from combinations of internal and external surface sensorarrays.

The sensor units provide broadband surface temperatures and the surfacetemperature fluctuations important to turbine component oxidation may beobtained. Arrays of the sensors provide broadband correlations betweentemperature fluctuations at different locations on an airfoil. Direct,broadband evidence of the location, stability and efficiency oftranspiration cooling jets may also be obtained.

The present invention also contemplates the production of two elementstrain sensors for use in gas turbines. A typical array is shown in FIG.5. The process steps for making the two-element strain sensor includethe six steps described above for the surface tempera ture sensorsexcept that the sputtering of the noble metal layer is done with twodifferent metals to form separately the first thin strip element 22 andthe second thin strip element 24. The first element 22 must have a largetemperature coefficient of resistivity relative to the second elementand is preferably platinum while the second element must have a largestrain coefficient of resistivity relative to the first element and ispreferably a platinum alloy containing 8-12 weight percent tungsten.Alternatively, the strain sensor may be constructed with a strainsensitive element as described and a sputtered Pt/Pt-Rh thermocouplelocated near the center of the strain sensitive element. To reduce therate of oxidation of the Pt-W alloy element above approximately l500F, aprotective layer of aluminum oxide or calcium stabilized zirconia isdeposited, preferably by sputtering, to a minimum thickness sufficientto protect the sensor element from the environment, e.g., to 0.1-0.5mil.

In addition to the utilization of the basic five-step procedure forproducing surface temperature sensors, surface strain sensors and asimple gas turbine component coating for protection against sulfidation,it may be utilized, with the addition of a step wherein an electricfield is superimposed across the oxide to prevent high temperatureoxidation. The field acts to cancel the electromechanical gradient whichoccurs naturally within the oxide and which provides the driving forcefor cation and/or anion motion in the oxide. The noble metal layer,preferably platinum, is the anode and the metallic coating is thecathode which is at the engine ground potential as shown in FIG. 6.Fields on the order of 10 volts/cm are sufficient to reduce the rate ofoxidation. In one test, it was experimentally ob served that a one voltpotential across a l u lcm) oxide significantly reduces the rate ofoxidation. A specimen having a FeCrAlY coating was prepared with No. 320grit blast and preoxidized for 24 hours at 2000F. Three 0.1 mil Ptelectrodes were sputtered on the oxidized surface and the specimen wasrcoxidized for 24 hours at 2000F. Positive and negative potentials wereapplied to two electrodes and the specimens were again oxidized. Afteran oxidation of 120 hours at 2000F in the presence of the electricfields, the specimens were cross sectioned and measurements were made ofthe oxides beneath each electrode including the electrode to which novoltage had been applied. The results indicated that with the platinumelectrode as the anode, a field of approximately 8 X volts/cm reducedthe rate of oxidation by a factor of two whereas with the platinumelectrode as the cathode, a field of approximately 1.2 X 10 volts/cmincreased the rate of oxidation by a factor of three.

It was determined that with the noble metal layer as the anode, the rateof oxidation decreases as the voltage increases until theelectrochemical gradient and the opposing electrical field balance andoxidation ceases. The voltage at which oxidation ceases should be thevoltage equivalent of the change in free energy of the oxidationreaction which, in the case of aluminum oxide, is approximately 2.1volts.

What has been set forth above is intended primarily as exemplary toenable those skilled in the art in the practice of the invention and itshould therefore be understood that, within the scope of the appendedclaims, the invention may be practiced in other ways than asspecifically described.

What is claimed is:

1. In a method for coating nickel-base, cobalt-base or iron-base gasturbine engine alloy substrates having an initial rare earth andaluminum-containing nickel, cobaltor iron-base alloy coatingapproximately 0.5-5.0 mils thick thereon, said initial coatingcontaining no more than 2%, by weight, rare earth metal andapproximately 5-257r, by weight, aluminum, the improvement whichcomprises:

mechanically working the surface of said initial layer to induceirregularity and angular topography in the aluminum oxide to beproduced; oxidizing said initial layer to produce an irregular aluminumoxide layer approximately 0.05-0.l mil thick to promote mechanicaladherence of a noble metal layer and to prevent alloying between saidinitial layer and said noble metal layer;

depositing a noble metal layer selected from the group consisting ofplatinum, rhodium, palladium and alloys thereof on said oxidized initiallayer to a thickness of approximately 0.1-0.2 mil; and

oxidizing said coated substrate to cause additional growth of saidoxidized initial layer to metallurgically insulate said noble metallayer from said substrate and said initial layer.

2. A method of coating an alloy substrate selected from the groupconsisting of the nickel-base, cobaltbase and iron-base gas turbineengine alloys comprisdepositing an initial rare earth andaluminumcontaining alloy layer on said substrate to a thickness ofapproximately 0.5-5.0 mils, said initial layer being an alloy selectedfrom the group consisting of, by weight, 2035% Cr, 15-20% Al, ODS-0.3%Y, balance Ni; 19-24% Cr, 13-17% A], 0.60.9% Y, balance Co; 25-29% Cr,12-14% Al, 06-09% Y, balance Fe; and 25% Cr, 15% Ni, 5% Ta, 5% A1, 0.1%Y, balance Co;

mechanically working the surface of said initial layer to induceirregularity and angular topography in the aluminum oxide layer to beproduced;

oxidizing said initial layer to produce an irregular aluminum oxidelayer approximately 0.05O.l mil thick to promote mechanical adherence ofa noble metal layer and to prevent alloying between said initial layerand said noble metal layer; depositing a noble metal layer selected fromthe group consisting of platinum, rhodium, palladium and alloys thereofon said oxidized initial layer to a thickness of approximately 0.1-0.2mil; and

oxidizing said coated substrate to cause additional growth of saidoxidized initial layer to metallurgically insulate said noble metallayer from said substrate and said initial layer.

3. The method of claim 2 wherein said mechanical working comprises gritblasting.

4. The method of claim 2 wherein said mechanical working comprisespeening.

5. The method of claim 2 wherein said initial layer is heated in air atapproximately 1900F for -3100 hours,

6. The method of claim 5 wherein said noble metal layer is deposited bysputtering.

7. The method of claim 6 wherein said noble metal layer is deposited inthe form of a thin strip array of thermoelectric junctions withthickened end portions suitable for use as terminal connections wherebysaid coating acts as a surface temperature sensor.

8. The method of claim 7 wherein said noble metal layer is deposited inan array of first and second thin strip elements, said first thin stripelement having a large temperature coefficient of resistivity withrespect to the second thin strip element and said second thin stripelement having a large strain coefficient of resistivity with respect tothe first whereby said coating acts as a surface strain sensor.

9. The method of claim 8 wherein platinum is deposited as the first thinstrip element and an alloy consisting essentially of 8-12 weight percenttungsten, balance platinum is deposited as the second thin strip element.

10. The invention of claim 9 wherein said second thin strip element iscoated with a protective oxide layer selected from the group consistingof aluminum oxide and calcium stabilized zirconia.

11. The method of claim 7 wherein said noble metal layer is deposited inthe form of a thin strip element having a large strain coefficient ofresistivity and a ther mocouple adjacent the center of the thin stripelement.

12. The method of claim 6 wherein an electric field is imposed acrossthe aluminum oxide layer to prevent further growth thereof, said noblemetal layer being the anode and said substrate being the cathodetherefor.

13. The method of claim 12 wherein a voltage potential of approximately2.1 volts is impressed across said aluminum oxide layer.

14. In a coating for the nickel-base, cobalt-base and to said firstmetal alloy layer, said aluminum oxide iron-base gas turbine enginealloys having a first rare layer having an irregular surface; and earthand aluminum-containing nickel-, cobaltor irona noble metal layerselected from the group consistbase alloy layer approximately 0.55.0mils thick, said ing of platinum, rhodium, palladium and alloys layercontaining up to 2%, by weight, rare earth metal 5 thereof approximately0.1-0.2 mil thick mechaniand approximately, 5-25%, by weight. aluminum,the cally bonded, by virtue of said irregular surface, to improvementwhich comprises: said aluminum oxide layer.

a layer of aluminum oxide 005-0.] mil thick bonded

1. IN A METHOD FOR COATING NICKEL-BASE, COBALT-BASE OR IRONBASE GASTURBINE ENGINE ALLOY SUBSTRATES HAVING AN INITIAL RARE EARTH ANDALUMINUM-CONTAINING NICKEL-, COBALT- OR IRON-BASE
 2. A method of coatingan alloy substrate selected from the group consisting of thenickel-base, cobalt-base and iron-base gas turbine engine alloyscomprising: depositing an initial rare earth and aluminum-containingalloy layer on said substrate to a thickness of approximately 0.5-5.0mils, said initial layer being an alloy selected from the groupconsisting of, by weight, 20-35% Cr, 15-20% Al, 0.05-0.3% Y, balance Ni;19-24% Cr, 13-17% Al, 0.6-0.9% Y, balance Co; 25-29% Cr, 12-14% Al,0.6-0.9% Y, balance Fe; and 25% Cr, 15% Ni, 5% Ta, 5% Al, 0.1% Y,balance Co; mechanically working the surface of said initial layer toinduce irregularity and angular topography in the aluminum oxide layerto be produced; oxidizing said initial layer to produce an irregularaluminum oxide layer approximately 0.05-0.1 mil thick to promotemechanical adherence of a noble metal layer and to prevent alloyingbetween said initial layer and said noble metal layer; depositing anoble metal layer selected from the group consisting of platinum,rhodium, palladium and alloys thereof on said oxidized initial layer toa thickness of approximately 0.1-0.2 mil; and oxidizing said coatedsubstrate to cause additional growth of said oxidized initial layer tometallurgically insulate said noble metal layer from said substrate andsaid initial layer.
 3. The method of claim 2 wherein said mechanicaLworking comprises grit blasting.
 4. The method of claim 2 wherein saidmechanical working comprises peening.
 5. The method of claim 2 whereinsaid initial layer is heated in air at approximately 1900*F for 70-300hours.
 6. The method of claim 5 wherein said noble metal layer isdeposited by sputtering.
 7. The method of claim 6 wherein said noblemetal layer is deposited in the form of a thin strip array ofthermoelectric junctions with thickened end portions suitable for use asterminal connections whereby said coating acts as a surface temperaturesensor.
 8. The method of claim 7 wherein said noble metal layer isdeposited in an array of first and second thin strip elements, saidfirst thin strip element having a large temperature coefficient ofresistivity with respect to the second thin strip element and saidsecond thin strip element having a large strain coefficient ofresistivity with respect to the first whereby said coating acts as asurface strain sensor.
 9. The method of claim 8 wherein platinum isdeposited as the first thin strip element and an alloy consistingessentially of 8-12 weight percent tungsten, balance platinum isdeposited as the second thin strip element.
 10. The invention of claim 9wherein said second thin strip element is coated with a protective oxidelayer selected from the group consisting of aluminum oxide and calciumstabilized zirconia.
 11. The method of claim 7 wherein said noble metallayer is deposited in the form of a thin strip element having a largestrain coefficient of resistivity and a thermocouple adjacent the centerof the thin strip element.
 12. The method of claim 6 wherein an electricfield is imposed across the aluminum oxide layer to prevent furthergrowth thereof, said noble metal layer being the anode and saidsubstrate being the cathode therefor.
 13. The method of claim 12 whereina voltage potential of approximately 2.1 volts is impressed across saidaluminum oxide layer.
 14. In a coating for the nickel-base, cobalt-baseand iron-base gas turbine engine alloys having a first rare earth andaluminum-containing nickel-, cobalt- or iron-base alloy layerapproximately 0.5-5.0 mils thick, said layer containing up to 2%, byweight, rare earth metal and approximately, 5-25%, by weight, aluminum,the improvement which comprises: a layer of aluminum oxide 0.05-0.1 milthick bonded to said first metal alloy layer, said aluminum oxide layerhaving an irregular surface; and a noble metal layer selected from thegroup consisting of platinum, rhodium, palladium and alloys thereofapproximately 0.1-0.2 mil thick mechanically bonded, by virtue of saidirregular surface, to said aluminum oxide layer.